1. Field of the Invention
This invention relates to an arbitrary layer tomographic method and an apparatus for carrying out the method. This invention particularly relates to an arbitrary layer tomographic method using at least one stimulable phosphor sheet, and an apparatus for carrying out the method.
2. Description of the Prior Art
In the computed tomographic apparatus (hereinafter referred to as a CT scanner) developed by Hounsfield et al., a tomographic image of a predetermined tomographic layer of an object is composed from a plurality of X-ray projection disctribution images obtained by exposing the tomographic layer of the object to X-rays in many different directions. As the CT scanner can provide a sharp tomographic image of a soft tissue, which could not be obtained with the convenitonal method using X-ray films, it has attracted much attention in the field of medical diagnosis.
Since the aforesaid CT scanner is intended to obtain a tomographic image of a predetermined tomographic layer of an object, it is impossible to obtain three-dimensional information over a wide range of a structure (e.g. an organ, bone, blood vessel, or the like) of the object by a single image recording operation using the CT scanner. In order to obtain three-dimensional information over a wide range of a structure of the object by use of the CT scanner, the image recording operation must be conducted several times for tomographic layers different from one another. However, when the image recording operation is conducted several times for the same object, the time required for the image recording becomes long. Further, deviations in position of the structure to be diagnosed occur among the tomographic images obtained by the respective image recording operations due to muscular motion, breathing motion, vermicular motion, or the like during the prolonged image recording time. As a result, it is not always possible to correctly diagnose the structure from the tomographic images.
As described in Math. Phys. KI. 69, pp. 262 to 277 (1917) by J. Radon, it has been mathematically verified that a two-dimensional object or a three-dimensional object can be unitarily reproduced from an infinite number of sets of projection data of the object. In the aforesaid CT scanner, a two-dimensional image is reproduced from one-dimensional projection image signals of a two-dimensional object obtained at various angles with respect to the object. Theoretically, however, it will also be possible to reproduce a three-dimensional image from two-dimensional projection image signals of a three-dimensional object obtained at various angles with respect to the object. This is called "three-dimensional reconstruction from radiographs" and there are a lot of technical reports thereabout. If a technique of reproducing a three-dimensional object from two-dimensional projection image signals of the object is developed, the technique will be very advantageous for medical diagnosis since it will become possible to obtain a tomographic image (two-dimensional image) of an arbitrary layer of a structure of the object by a single image recording operation.
However, in order to use the detector of the CT scanner as a two-dimensional sensor for obtaining a tomographic image of an arbitrary tomographic layer of a structure of an object, a very large number of detection elements must be positioned two-dimensionally. It takes a very long time for image signals to be transferred from the two-dimensional sensor comprising many detection elements and, consequently, the time required for the image recording becomes long. An increase in the image recording time presents a very serious problem in a method of composing an image having a given number of dimensions from a plurality of projection image signals having dimensions lower by one dimension than the dimensions of the image. Namely, when a stationary image of the head, the skeleton, or the like is recorded, no serious problem is presented even if the image recording time is prolonged. However, when an image of an organ is recorded, the condition of the organ changes due to muscular motion, breathing motion, vermicular motion, or the like during the prolonged image recording time. As a result, the contrast and the spatial resolution of the tomographic image ultimately obtained are adversely affected, or noise called artifact occurs, making it impossible to obtain a tomographic image suitable for viewing and diagnostic purposes. When the detector, which is employed in the CT scanner, is used as the two-dimensional sensor, the time required for the image recording becomes too long to obtain an image suitable for viewing and diagnostic purposes. Therefore, there has not heretofore been any practicable apparatus for obtaining image information representing a three-dimensional image of an object, thereby obtaining a tomographic image of an arbitrary tomographic layer of the object.
In general (also in the conventional CT scanner), in order to achieve high diagnostic accuracy and efficiency, the tomographic image should exhibit a spatial resolution sufficient to permit discrimination of details of the tissue. The spatial resolution of the tomographic image ultimately obtained depends on the number of detection elements of the detector per unit space. However, when detection elements employed in the CT scanner (for example photomultipliers provided with scintillators on light receiving faces) are used as the detection elements, a sufficient spatial resolution cannot be obtained because such photomultipliers have a large size and the number thereof per unit space of the detector becomes small. High-pressure xenon gas detection elements and semiconductor detection elements can be positioned more densely than photomultipliers. In this case, however, the spatial resolution that can be realized is at most about 1 line/mm. Furthermore, high-pressure xenon gas and semiconductor detection elements exhibit a lower sensitivity than photomultipliers provided with scintillators on light receiving faces. Therefore, to obtain a tomographic image of the same quality as when photomultipliers are used, it is necessary to increase the radiation dose which the object receives. Further, when the detector is composed of a plurality of detection elements, the respective detection elements should exhibit the same sensitivity. However, from the technical viewpoint, it is not always possible to make the detection elements exhibit exactly the same sensitivity.